Hillary Rodham Clinton lost her last best chance to score an upset on Barack Obama's turf Tuesday, putting the Illinois senator a step closer to becoming the country's first black presidential nominee. Obama was the long-standing favorite in North Carolina, and he won with the overwhelming support of black voters there despite an intense effort by Clinton to turn the state around. Obama's victory there was tempered by the fact that Clinton beat him handily among white voters, extending her argument to superdelegates who will decide the nomination that she will be the stronger general-election candidate. ...Eighteen percent of North...